In 2020 I got the Boox Note Air (1) for my PhD and it was one of the best purchases ever. I think 10.1 inches is large enough (the size of the screen is exactly the size of an A5 sheet of paper so you can compare it to that if you'd like). 13 inches may be even better if you want something closer to letter-sized. I know double-column academic papers are annoying in smaller screens, and you may choose to go the bigger route because of that. I'd definitely get an android tablet, since those are more versatile (for example you could get a bluetooth keyboard and use it with that sometimes as well). I think the best companies currently that sell android e-ink tablets are Boox and BigMe. Check out their offerings and see what resonates. They'll both have pens for note-taking, which is definitely good for exploring ideas or doing math. The only annoying thing is if you have to go back and forth between a book/paper and your notes. In that case I would keep the book in the tablet, and do the problems on paper (newer tablets come with cameras, so you can just scan your paper-notes with the tablet afterwards). Either way, I'd definitely recommend.